In a charge transfer unit, for example, a CCD unit of a conventional solid-state imaging device, a transfer electrode that transfers a charge is provided on an impurity layer, formed in a surface area of a semiconductor substrate, with an oxide film interposed between them. In the CCD unit, the impurity layer is formed in the surface area of the semiconductor substrate below each transfer electrode by implanting, for example, an n-type ion such that a profile is deepened toward a charge transfer direction. An electric potential deepened toward the charge transfer direction is formed below each transfer electrode by the impurity layer having the profile described. Accordingly, the charge can efficiently be transferred in the CCD unit.
It is well known that the impurity layer can be formed using a so-called grating mask in which plural transmission regions are formed in which a light transmittance gradually changes. The grating mask is formed by a dot pattern, in which dot-shaped light shielding portions (hereinafter referred to as “dot”) through which the light is not transmitted are regularly arrayed on a surface of a glass plate through which the light is transmitted. In the dot pattern, an area of each dot is changed while the number of dots per unit area is kept constant, thereby changing the light transmittance. The grating mask controls the light transmittance by the area of each dot constituting the dot pattern.
Alternatively, the grating mask may be formed by a dot pattern, in which dot-shaped translucent portions through which the light is transmitted are regularly arrayed on a surface of a glass plate through which the light is not transmitted. Accordingly, the term of “dot pattern” means the dot patterns in both the above-described grating masks.
The shape of the dot is not limited to the square, but the dot may be formed into a polygonal shape or a circular shape.
A method for forming the impurity layer using the grating mask will be described below. A resist material is evenly applied on the semiconductor substrate. Then the resist material is exposed using the grating mask. When the exposed resist material is developed, a resist-remaining film is thinned in a region having the higher light transmittance. Accordingly, the plural resist films whose film thicknesses are thinned toward the charge transfer direction are repeatedly formed on the semiconductor substrate. When ions are implanted into the semiconductor substrate through the resist film as a mask, the impurity layer that is deepened toward the charge transfer direction can be formed by the ion implantation process with one step.
However, in the impurity layer formed in the above-described process, the inventors of the present invention found that a potential dip is formed in a step portion of a stepwise potential profile, and therefore charge transfer efficiency is degraded. The problem prevents further improvements in the device.
In view of the foregoing, an exposure mask that can improve the charge transfer efficiency, a method for manufacturing the semiconductor device having the impurity layer and a solid-state imaging device are provided.
In accordance with an aspect of the invention, an exposure mask includes a first transmission region, a second transmission region and a third transmission region that is located between the first transmission region and the second transmission region. In the exposure mask, light transmittances of the first transmission region and the second transmission region discontinuously change in a boundary portion between the first transmission region and the second transmission region. The first transmission region includes a first dot pattern where light is shielded or transmitted, and the first dot pattern includes an array of plural dots. The second transmission region adjacent to the first transmission region includes a second dot pattern, and the second dot pattern includes an array of plural dots. In the second dot pattern, an area of the dot is larger than that of the dot of the first dot pattern. The third transmission region includes a third dot pattern, and the third dot pattern includes an array of plural dots. In the third dot pattern, the dot has an area intermediate between the dot of the first dot pattern and the dot of the second dot pattern.
In accordance with another aspect of the invention, a method for manufacturing a semiconductor device having an impurity layer includes the steps of applying a resist material on a surface of a semiconductor substrate, exposing the resist material, developing the exposed resist material and forming the impurity layer. In the step of exposing the resist material, the resist material is exposed using the exposure mask described. In the step of forming the impurity layer, the impurity layer is formed by implanting ions into the semiconductor substrate from above a resist film formed by developing the resist material.
In accordance with still another aspect of the invention, a solid-state imaging device includes a first-conductive-type well that is formed on a surface of a semiconductor substrate, a first impurity layer, a second-conductive-type second impurity layer and a first electrode. The first impurity layer is formed on a surface of a well to act as a photodiode. The second impurity layer is formed on the surface area of the well apart from the first impurity layer in a charge transfer direction. The second impurity layer includes a front-step portion that is deepened toward the charge transfer direction and a rear-step portion that is deepened toward the charge transfer direction with a gradient larger than that of the front-step portion. The first electrode is formed on the second impurity layer and on the well between the second impurity layer and the first impurity layer while an insulating film is interposed between them.
An exposure mask, a method for manufacturing a semiconductor device having an impurity layer and a solid-state imaging device according to embodiments of the present invention will be described in detail with reference to the drawings.